1. Field of the Invention
The present invention relates to an AE/ultrasound detection system. For example, it relates to a technique for detecting AE/ultrasound with the use of an FBG sensor, and to a system in which such technique is applied for evaluating the soundness of a structure.
2. Background Art
In recent years, for the purpose of improving the reliability of a structure, it has been expected to establish a soundness evaluation apparatus. When the soundness of a structure is evaluated, it is very important to measure strain and to detect defection such as cracking. Strain has been often measured with the use of a resistive strain gauge utilizing change in electrical resistance associated with deformation of metal. Further, as a method for detecting defection such as cracking, the detection of elastic-wave emission (AE: acoustic emission) associated with occurrence of defection and a nondestructive inspection utilizing ultrasound are conducted.
Piezoelectric elements have been widely used as AE detection sensors and ultrasound detection sensors in a nondestructive inspection using ultrasound. However, such AE/ultrasound measurement using piezoelectric elements involves the following problems. Namely, since piezoelectric elements are influenced by electromagnetic interference, the AE/ultrasound measurement cannot be conducted in the atmosphere of electromagnetic waves. Also, since piezoelectric elements have narrowband response frequency characteristics, it is necessary to change the kind of piezoelectric elements in accordance with the frequency band of the AE/ultrasound to be detected.
In recent years, in order to solve the above problems of the AE/ultrasound detection using piezoelectric elements, AE/ultrasound detection using an FBG (Fiber Bragg Grating) sensor, a kind of optical fiber sensor, (which will be occasionally referred to simply as an “FBG” hereafter in the specification) has drawn attention. Conventionally-proposed AE/ultrasound detection systems using the FBG are divided roughly into two kinds, depending on the light source used. One kind is a system using a laser light source, as disclosed in JP Patent Publication (Kokai) No. 2005-326326 A. In this system, laser light whose oscillation wavelength is set at a wavelength at which the reflectance of the FBG is decreased by half is caused to enter the FBG. Upon entry of the light, since the intensity of the light reflected from the FBG varies depending on the AE/ultrasound received by the FBG, the AE/ultrasound can be detected. The other type is a system using a broadband light source, as disclosed in JP Patent Publication (Kokai) No. 2005-009937 A. In this system, broadband light including a Bragg wavelength, the central reflection wavelength of the FBG, is caused to enter the FBG, and the reflected light is transmitted to an optical filter having the transmission wavelength band approximately equal to the reflection wavelength band of the FBG. At this point, a portion of the transmission wavelength region of the optical filter needs to overlap the reflection wavelength region of the FBG. By utilizing such phenomenon; that is, the transmitted-light intensity or the reflected-light intensity of the optical filter changes depending on the AE/ultrasound received by the FBG, the AE/ultrasound can be detected.
However, the Bragg wavelength of the FBG fluctuates depending on the temperature and strain received by the FBG. For example, in the case of a FBG having a Bragg wavelength of 1550 nm generally used for evaluating the soundness of a structure, the Bragg wavelength varies by 10 pm for a temperature of 1° C. with respect to the reflection wavelength band from 200 pm to 2000 pm, and the bragg wavelength also varies by 1.2 pm per micro strain. Thus, when the FBG receives a large change in temperature and strain, based on the system using a laser light source, there are cases in which the laser oscillation wavelength does not fall within the reflection wavelength region of the FBG. Similarly, based on the system using a broadband light source, there is a situation in which the transmission wavelength region of the optical filter does not cross the reflection wavelength region of the FBG. In such cases, the AE/ultrasound received by the FBG cannot be detected. Thus, it is necessary to control the laser oscillation wavelength or the optical-filter transmission wavelength in accordance with the fluctuation of the Bragg wavelength of the FBG. However, when the Bragg wavelength of the FBG fluctuates at high speed, such control cannot follow the change of the Bragg wavelength, and it is therefore conceivable that the AE/ultrasound cannot be detected. Particularly, since the AE generated upon destruction of a material is a phenomenon caused along with a large change of strain, a large fluctuation of the Bragg wavelength is caused upon occurrence of the AE. Thus, it is conceivable that the AE detection is very difficult with convention measurement systems.
In order to overcome the disadvantage, JP Patent Publication (Kokai) No. 2003-169801 A discloses a technique in which two thermally-coupled Bragg gratings made of the same material are used, so that the ultrasound received by the Bragg gratings can always be detected. In this technique, the Bragg gratings of the sensor and the filter that are thermally-coupled are synchronized upon receiving a temperature change, and the Bragg wavelengths are thus fluctuated.